Meshing Thin Vents in Castings

Figure 1: Simple cylindrical casting
with overrun and thin vent

Figure 2. Two dimensional cross-section
of mesh and open volume with thin vent

Figure 3. Two dimensional cross-section
of mesh and open volume with larger vent

Question: I have very thin vents in my casting, and they're causing my mesh to get out of hand…what can I do?

In many designs for high pressure die castings, the dies have vents attached to overrun chambers. These vents are intended to allow air to escape the cavity during the short duration of the filling process. Since these vents are only intended for the release of air, they are typically made extremely thin. Moreover, they may have odd shapes or follow a circuitous path, such as to avoid cooling channels.

Trying to resolve these very thin vents in a computational mesh may require a very large number of cells. It's useful to remember, however, that the vents are only there for the escape of trapped air. It is not necessary to model their exact shape or size. In fact, all that is necessary is to create an open channel from the overrun to the boundary of the mesh, where you will specify a pressure boundary condition and a fluid fraction of 0. This channel can have any shape. Moreover, it is not even necessary to match the cross-sectional areas. This is because when you are modeling castings in FLOW-3D you are using its one-fluid model, which does not simulate the air movement.

A simple example helps demonstrate the benefits of this approach. Figure 1 shows a simple cylinder with an attached overrun and vent. The cylinder is 4 cm in radius and the vent is 0.7 mm thick. Figure 2 shows a 2-D cross-section through the cylinder and overrun with the vent exactly represented. A fixed point is defined in the middle of the vent to resolve the vent on this scale. Otherwise, even smaller cells (and a larger overall mesh) would be required. As drawn, this mesh contains 49,000 cells.

Now let's make a change to the vent, doubling its size, (see Figure 3). No extra fixed points are required, and the mesh is only 39,000 cells, a savings of 20% from the prior mesh. In a larger casting, the savings can be even more significant.

Another option available to users is to model the problem using a valve with a specified loss coefficient in place of the vent. In this case, at the location of the vent in the die, the user would specify a valve using the Initial tab in the GUI's Model Builder panel and provide a loss coefficient based on the diameter and length of the valve (see the “How do I Model Valves?” Section of the Users Manual). With this approach, the user need not mesh the vent at all.

This is a good example of the importance of asking, “What do I want to learn from this simulation?” By realizing that the flow of air through the vent is not important, it is possible to reduce the resolution of the mesh (or eliminate it entirely), decrease run time and solve your die casting problem more quickly.